Well-quasi-ordering and Embeddability of Relational Structures

At the end of the forties, Fraïssé, following Cantor, Hausdorff and Sierpinski, highlighted the role of the embeddability quasi-order in the theory of relations. Since then, many results illustrating this role have been obtained (a large account was included in Fraïssé’s book Theory of Relations). In this paper, I present a selection of results centered on the notion of well-quasi-order (wqo). I mention several problems on wqo and hereditary classes of relational structures; some of these problems go back to the seventies.